The common enterobacterial antigen has been purified only recently in three different laboratories. The immunogenicity of these preparations is being studied in rabbits, guinea pigs, and several strains of mice. The humoral antibody response is measured by means of the hemagglutination procedure and the cellular response by skin tests and H3 thymidine incorporation by lymphocytes. In additional experiments, the mitogenicity and polyclonal activation capacity of these preparations are being investigated.